Commonly, an electrophotographic copying appararus of transfer type includes a photoreceptor, for example, in the configuration of a drum having a photconductive layer or light-receiving surface on a conductive backing therearound and rotatably mounted below a transparent platform for placing an original to be copied thereon, so as to cause the photoconductive photoreceptor surface to sequentially pass various processing stations disposed therearound. These stations include: a corona charging station for preliminarily charging the photoreceptor surface; an exposure station for exposing the thus charged photoreceptor surface to image light of the original directed thereto, an optical system and illuminating light source disposed between the platform and the photoreceptor drum, so as to form an electrostatic latent image of the original on the photoreceptor surface; and a transfer station having a transfer unit for transferring the latent image onto a transfer material or copy paper, either directly or after having been developed into a visible toner image by a developing unit depending on the types of the copying apparatus in a known manner. Some of such copy apparatuses are provided with horizontally, reciprocatingly movable platforms and stationary optical systems, while others have movable optical systems with stationary platforms for scanning the original to be copied in the latent image formation. The transfer material is fed, through transportation rollers, onto the photoreceptor surface having thereon the thus formed latent image and is subsequently presses against the photoreceptor surface by transfer rollers of the transfer unit disposed adjacent to the photoreceptor surface for the latent image transfer. In this case, however, since the copy paper closely adheres to the photoreceptor surface due to electrostatic attraction developed therebetween, it is necessary to separate the copy paper from the photoreceptor surface after transfer.
For effecting the above described copy paper separation, there has conventionally been proposed a separation device in which separation claws are employed. The front edges of the separation claws are directed against the direction of the advancing copy paper to contact the photoreceptor surface for mechanically stripping the copy paper from the photoreceptor surface. This device, however, while simple in construction and compact in size, has such disadvantages that the photoreceptor surface tends to be damaged during repeated use due to the contact of the front edges of the claws with the photoreceptor surface. This not only adversely affects the quality of copied images, but results in short life of the photoreceptor itself. Accordingly, in order to reduce the damage to the photoreceptor surface to a minimum level, the separating claws must be arranged so as to contact the photoreceptor surface only during copy paper separation instead of contacting the same at all times, and the material and shape of such claws must be fully taken into account to minimize the damage to the photoreceptor surface even during the contact thereof with the photoreceptor surface. For meeting such requirements, separating claws of different configuration associated with various operating mechanisms have also been presented some these, the front edge of the claw is made as thin as a doctor's scalpel for increasing the separating effect, thus simultaneously reducing the contact area with respect to the photoreceptor surface, or the claw is movably associated with a cam which rotates in association with the copying operation for causing the same claw to contact the photoreceptor surface in timed relation only during the copy paper separation. Each of these conventional arrangements, however, is not necessarily favourable and still damages the photoreceptor surface, especially in the photoreceptor employed in the latent image transfer type copying apparatus, to such an extend that such arrangements are not suited to practical use.
Another problem encountered when such separating claws are employed in the electrostatic latent image transfer type copying apparatus in the adverse effect on the copied images due to tribo-electrical or frictional charge arising from frictional contact between the separating claws and the copy paper. In other words, since the copy paper used for such latent image transfer type copying apparatus is provided with an insulating dielectric surface layer composed, for example, of polyvinyl acetal, polyurethane, copolymer of vinyl acetate and acrylic ester and the like, frictional charge builds up upon contact of the surface of the copy paper with the under surface of the separating claw during the copy paper separation, and the background portion of the latent image is charged to different polarities depending on the materials of the separating claws. For example, the background has a negative polarity if the material of the claw is of acrylic, polyester or Mylar(commercial name for polyethylene glycol terephthalate film produced by Du Pont), or has a positive polarity if the material of the same is of Teflon(commercial name for polytetrafluoroethylene product by Du Pont) and the like. Accordingly, if the material of the claw is one which charges the background portion of the copy paper with the same polarity as that of the latent image, the portion thus charged by the contact with the claw appears as unsightly contact traces after development. When the copy paper is not picked off the photoreceptor surface immediately after the passage of the grounded transfer roller of the transfer unit, the transfer is effected on the background portion of the latent image wherein no transfer takes place normally, thus resulting in soiling in the background of the copy after developing. This defect is considered to be due to the fact that the charge which builds up in an electroconductive base of the copy paper due to the charge in the background portion of the latent image on the photoreceptor is not sufficiently erased by the grounded roller during separation of the copy paper from the photoreceptor surface. Even when the copy paper is adapted to be forcibly separated from the photoreceptor surface by the separating claw immediately after the passing of the grounded roller for the prevention of the above described disadvantage, the surface of the copy paper tends to strongly contact the under surface of the separating claw, and the above defect becomes all the more conspicuous. Accordingly, when employing copy paper separation claws in an electrostatic latent image transfer type copying apparatus, countermeasures not only for the damage to the photoreceptor surface by the separating claws, but also for the adverse effect on the copied images due to tribo-electrical charge of the separating claws and the copy paper must be taken into consideration.